Key vocabulary Premise -- claim or assumption that forms the basis of an argument. True -- refers to an objective fact or reality; used to describe a premise or conclusion. Valid -- describes an argument that is logically coherent (i.e. the conclusion follows from the premises) Note: a valid argument is not necessarily true: e.g. ‘Apes are insects. Chimps are apes. Therefore, chimps are insects.’
Deductive Reasoning Moves from the general to the particular. Usually relies on syllogism (using two premises to deduce a logical conclusion): All dogs are mammals. Buster is a dog. Therefore, Buster is a mammal. Reliability requires true premises & valid arguments.
Inductive Reasoning Moves from the particular to the general. Uses repeated observations to reach a general conclusion. Good generalizations require: number, variety, coherence Seek out counter-examples; it only takes 1 to disprove ‘facts’ determined by induction: e.g. ‘All birds can fly.’
Deduction vs. Induction Particular general Hawks can fly. Eagles can fly. Pigeons can fly… All birds can fly. Generally considered less certain, but more informative. Key concern: hasty generalizations General particular All birds can fly. A penguin is a bird. Penguins can fly. Appears more certain, but premises are often based on induction. Key concerns: false premises invalid syllogism
Fallacies: invalid patterns of reasoning Ad hominem / appeal to authority literally ‘against the man’ Attacking or supporting an argument based on the person presenting it rather than the argument itself. e.g. Ms. Beck says… ‘So what? She’s an idiot!’ Post hoc ergo propter hoc literally: ‘after this, therefore’ confusing correlation with causality. e.g. Many teachers are smart… Teaching must make ppl smart.
‘Appeal to force or fear’; suggesting that unfavorable results implies faulty reasoning e.g. ‘I couldn’t have failed; my parents will be furious if I fail another test…’ Ad misericordiam / special pleading ‘Appeal to pity’; using emotion to justify double standards/making exceptions for ‘special cases’. e.g. ‘I know it’s wrong to cheat, but I’ll never get into college if I don’t.’ Loaded question/language A question or statement with a built- in assumption e.g. ‘Do you always cheat on exams?’ ‘The best students will agree that… ’ Ad bacculam / unpalatable consequences
Invalid syllogism Failure of logical reasoning in an argument. e.g. (see cartoon) Equivocation Using different definitions of the same word (ambiguous language) e.g. ‘A pound is better than nothing. Nothing is better than love. A pound is better than love.’ Ad ignorantiam Claiming something is true because there is no evidence that it is not true. e.g. ‘Unicorns must exist, because we can’t prove that they don’t exist.’
Hasty generalizations e.g. ‘All birds can fly.’ False dilemma a.k.a. binary thinking Presenting a complex situation as if there were only two options (a black or white scenario). e.g. ‘You’re either with us or against us.’ False analogy Presenting two different ideas as similar. e.g. ‘Just as the rain wears down the mightiest mountain, hard problems can be solved with patience & persistence.
Circular reasoning Assuming the truth of something you are supposed to be proving. e.g. ‘Bobby gets all A’s, he must be the smartest kid.’ ‘How do you know he’s the smartest kid?’ ‘Because he gets all A’s. ’ Appeal to common practice Implying that because something is tradition or popular, it must be correct. e.g. ‘There’s nothing wrong with coming late to class; everybody does it. ’; ‘But we’ve always done it this way...’